Spray-drying is the standard method for manufacturing laundry detergent base powder. Typically, detergent ingredients are mixed together to form an aqueous detergent slurry in a mixer, such as a crutcher mixer. This slurry is then transferred along a pipe through a first low pressure pump and then through a second high pressure pump to a spray nozzle. Between the first and second pumps, a liquid detergent ingredient is added. From the second pump, the slurry is then sprayed into a spray-drying tower, and spray-dried to form a spray-dried powder. The slurry in the pipe between the high pressure pump and the spray nozzle is at high pressure.
Consumers are used to dosing a particular quantity of detergent powder into the wash. If the density of the powder is too high then they will tend to overdose, since the same volume of high density powder contains more detergent active material than an equal volume of low density powder. Thus, it is desirable to produce laundry detergent powder with a low density.
Traditionally, nitrogen-rich gas is injected into the high pressure pipe between the high pressure pump and the spray nozzle. The nitrogen-rich gas is mixed with the slurry to form a slurry containing nitrogen-rich gas bubbles. Thus, the nitrogen-rich gas bubbles are dispersed in the high pressure slurry and trapped within the spray-dried slurry droplets during drying and subsequently trapped in the spray-dried powder.
However, a problem with this method is the low efficiency of entrapment of the nitrogen-rich gas bubbles. Thus, to achieve low densities large volumes of nitrogen-rich gas need to be pumped into the high pressure slurry. This is inefficient and expensive due to the equipment required to achieve pumping of the large volumes of nitrogen-rich gas.
Thus, there is a need in the art to improve the efficacy of nitrogen-rich gas capture to achieve low density detergent powders.
The Inventors have surprisingly found that introduction of the nitrogen-rich gas into the slurry between the first and second pumps, increases the efficiency of bubble entrapment and so results in lower density spray-dried detergent powder at much lower injected levels of nitrogen-rich gas.